Detailed study of sulfide scaling at la courneuue nord, a geothermal exploitation of the Paris Basin, France

Abstract The fluid of the Dogger aquifer is always used through a closed loop formed by the production well, the heating plant and the injection well. After two or three years of exploitation of the geothermal doublets in the northern part of the Paris basin, scaling and plugging problems have appeared in some cases. The results of the detailed study carried out at La Courneuve Nord, a typical site of this area, are presented. The drawdrown of production rate, scaling in the heat exchanger and the increase of injection pressure required a rapid decision for workover operations on the wells. These cleaning operations and joint research studies allowed us to identify the cause of the plugging as well as to locate these deposits and to estimate their importance. Iron sulfide was found from the base of the production well casing all the way to the injection well casing. The thickness of this scale induces hydraulic pressure loss along the casings. Moreover, part of this sulfide is carried away by the fluid and fills up the bottom of the open hole injection well. After the cleaning operations, the hydraulic properties of the reservoir seem to be recovered. Chemical and mineralogical analyses of theses deposits identified the presence of a large variety of iron sulfide (mackinawite, pyrite, pyrrhotite) and a typical corrosion product (Fe 2 (OH) 3 Cl). Biochemical and bacteriological studies show a very high content of micro-organisms. At the end of the workover operations, several geophysical logs gave information on the corrosion of the casing as well as the residual scale. A balance of the iron content is calculated, between the volume of deposits and the iron removed by the corrosion of the casing. A chemical model, TPDEGAZ, is used to calculate the evolution of the saturation indexes of the fluid with respect to iron sulfide phases. The effects of parameters such as pressure, temperature, degassing and addition of iron by corrosion are simulated. The results of the observation and modeling approaches are compared.